1. Field of the Invention
This invention relates to stitching systems utilizing a bobbin from which thread is drawn and, more particularly, to a thread tensioning assembly used in conjunction with the bobbin which produces controlled resistance to the payout of thread from the bobbin.
2. Background of the Invention
In sewing operations, and particularly in embroidery sewing operations, the tension of two source components forming the lockstitch needle thread and bobbin thread must be balanced to achieve a high quality stitch. If the tension of the needle thread is significantly greater than the bobbin thread tension, the bobbin thread can be pulled from through the underside of the fabric and show at the top side of the fabric being sewn. This condition can cause puckering of the fabric or disfigured sewing to occur. If the needle thread tension is significantly less than the bobbin thread tension, loops can form on either side of the fabric and the stitch formation can appear loose or distortedly large.
A primary job of the sewing equipment operator is to keep bobbin and needle thread tensions as close as possible to balanced. The method of balancing thread tension has historically been carried out by having the sewing operator observe the sewing pattern after stitches are laid down. Good sewing operators constantly adjust the tension of both needle and bobbin threads to maintain the proper balance. Less skilled operators may not consistently maintain this balance as a result of which poor quality stitch formation may result.
The task of balancing tensions is further complicated by the inherent problems of conventional sewing hooks and bobbin cases. The "all steel" hook construction requires oil to maintain cool and consistent rotation. However, it is unavoidable that hook rotation becomes inconsistent because the oil can dissipate from the hook raceway track as a result of which friction between the bearing surfaces of the bobbin basket and hook raceway increases. In a typical operating shift, the all steel hook construction may enter a "stick-slip" cycle, which is triggered by friction. When the hook sticks, maintenance of good stitch formation requires that the operator tighten the needle thread tension for the loop to be pulled past the thread escape exit and up into the fabric being sewn. If the tension is not tightened during the stick mode, loops may form on the material being sewn. After the tightening adjustment is implemented, the "slip" mode can return. However, the previous tightening adjustment of needle thread tension may cause the tension to be too tight for the current "slip" condition. As a result, puckering of the fabric or bobbin thread coming through the top of the fabric may occur. The operator thus must continuously monitor the thread tension balance brought about by the "stick-slip" cycle.
It has been observed that the "stick-slip" cycle tends to cause the thread tension balance point to progressively migrate upwards as the operator tightens needle thread tension to compensate for the "stick mode". Often the operator tries to strike the proper balance by tightening the bobbin case spring to reduce the rate of bobbin spooling. The net effect is the equilibrium balance point creeps to a higher tension for both needle and bobbin threads. The higher equilibrium point is detrimental to sewing performance because higher tension, even if balanced, may cause increased incidence of thread breakage and puckering. Higher tensions cause thread to pucker the fabric as it assumes its reflexive state. Lower tension at equilibrium is optimal because thread is less vulnerable to breakage from sharp burrs, edges, or interruptions along the thread path.
The monitoring task becomes even more burdensome in embroidery where the operator is responsible for multi-head machines often having from 15 to 20 sewing heads per machine and 9 to 12 needles per head. The embroidery machine often has 3 tension gauges per needle. Therefore, the operator may be responsible for 720 needle tension gauges per machine for a 20-head, 12-needle machine. It is virtually impossible for the operator to regulate this many variables to achieve thread tension balance for optimal stitch formation.
The task of balancing needle thread with bobbin thread can be further complicated by the variation of the bobbin case settings. Presently the bobbin is placed in the bobbin case and thread routed between the side wall and bobbin case spring. The conventional bobbin case has a regulator screw that increases or decreases the spring pressure to affect the spooling of the bobbin thread. The operator is taught to set the regulator screw to a pressure that allows the bobbin to spool a predetermined number of inches when he/she lets the bobbin case drop, using a slight jerk of the wrist. Unfortunately this is not a precise and quantifiable procedure and the bobbin spring tension settings tend to vary greatly. Often during the course of a month, week, day, or even embroidery run, the bobbin case spring can lose tensile strength or flex.
The setting of the bobbin case tension is the first procedure for correctly balancing thread tension. While it was shown above that there could be several hundreds of settings for adjusting needle thread tension, there is only one bobbin case setting per sewing head. All needle thread balancing is calibrated based on a definitive and stable bobbin case setting. Once the proper bobbin case setting is made, all balancing between the needle thread and bobbin thread tensions is implemented from the needle thread tension gauges.
If the bobbin thread tension varies throughout its run or after its changing, the required adjustment of the large number of needle tension gauges goes up exponentially. An operator cannot react effectively to the constantly changing needle thread tension and bobbin thread tension combinations.